1. Field of the Invention
This invention relates in general to a process for improving the image quality in picture transmission of a facsimile system.
2. Description of the Prior Art
In facsimile transmission for example for long distance office copiers, weather map transmission or transmission of other documents by wire or wireless, the picture quality of the transmitted image suffers due to the imperfections of the transmission channel and the limitations in availability of sufficient bandwidth.
A frequently occurring example is that only black and white information transfer is required for letters, line drawings and the like. The picture signal which is obtained from the original by electro-optically scanning consists of a so-called two level signal which means that the signal has only two states which according to an arrangement the signal can either be black or white. If within the original there are only short lines of black, the scanner will also send out very short pulses which, however, in the process of transmission in the communication channel are distorted and at the receiver end arrive without noticeable amplitude and therefore their information cannot be recovered. If one tries to provide at the receiver a suitable amplifier so as to raise the signals over the recognition level a series of errors are introduced and echoes and other noise pulses distort the image information which makes this approach unsuitable. In present transmission techniques, a process of bandwidth reduction is used so as to save communication channel bandwidth or so as to increase the transmission speed and the two level signal is changed to a three level signal and transmitted as a three level signal. Such apparatus is described for example in U.S. Pat. No. 3,303,284 and in German Pat. No. 1,762,644.
The three level signal is obtained from the two level signal by inverting the signal whenever a change of information occurs which is to say that whenever the two level signal changes the result is a three level signal. The signal values of +/- and 0 will be experienced whereby in accordance with prior arrangement +/-1 is represented by the black areas and 0 is represented by the white areas of the original or vice versa.
During the resulting reduction to 1/2 of the base frequency in the three level signal case, the created signal can now be either transmitted at half the band width or twice the speed. At the receiver, the signal is converted again into a two level signal and recorded. However, this process has the problem that short black white lines of an original which were already reduced in their amplitude by the scanner aperture or lines which are wide however are below the trigger level in amplitude and will be lost because the amplitude of these signals will not be sufficient to invert the two level signal into a three level signal.